1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell. The fuel cell has an anode and a cathode. A fuel gas is supplied to the anode, and an oxygen-containing gas is supplied to the cathode for inducing electrochemical reactions at the anode and the cathode, and electricity generated in the electrochemical reactions is supplied to a load.
2. Description of the Related Art:
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between a pair of separators. The membrane electrode assembly and the separators make up a unit cell for generating electricity.
FIG. 8 is a view schematically showing a fuel cell system 2 including such a fuel cell stack 1 (see Japanese Laid-Open patent publication No. 2002-93438). In the fuel cell system 2, a fuel gas such as a hydrogen-containing gas is supplied to a humidifier 4 through an ejector 3. After the fuel gas is humidified to have a predetermined humidity, the humidified fuel gas is supplied to the anode 5. The catalyst of the anode 5 induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode 7 through the electrolyte membrane 6, and the electrons flow through an external circuit to the cathode 7, creating a DC electric current supplied to a load. An oxygen-containing gas such as air is supplied to a humidifier 8. After the oxygen-containing gas is humidified to have a predetermined humidity, the humidified oxygen-containing gas is supplied to the cathode 7. At the cathode 7, the hydrogen ions from the anode 5 combine with the oxygen in the oxygen-containing gas to produce water. After the oxygen in the oxygen-containing gas is partially consumed in the chemical reaction, the oxygen-containing gas and the water vapor produced in the chemical reaction are discharged as the exhaust gas from the fuel cell stack 1.
The humidifiers 4 and 8 are used for humidifying the hydrogen gas and the air to keep the electrolyte membrane at a predetermined humidity suitable for power generation. If the water is trapped in the passage near the anode 5, the hydrogen gas may not be sufficiently supplied to the anode 5, and the voltage drop may occur undesirably. In an attempt to solve the problem, a valve 9 is provided in the circulation passage of the hydrogen gas. The valve 9 is opens in a certain condition for discharging the exhaust gas containing water so that the water is not trapped in the passage, and the desired voltage can be maintained.
The unconsumed hydrogen in the exhaust gas is wastefully discharged from the anode 5 to the outside. The loss of the unconsumed hydrogen lowers the fuel economy. Thus, it is necessary to suppress the hydrogen concentration in the hydrogen gas (exhaust gas) below a predetermined level. The optimum condition for discharging the minimum amount of the hydrogen gas needs to be determined empirically through various experiments. Further, in the conventional fuel cell system, a mechanism for decreasing the hydrogen concentration in the exhaust gas, a mechanism for diluting the hydrogen gas, and a mechanism for combusting the hydrogen gas are required.